Good compression ratio

[DartVadar]

So I'm rebuilding my 360, and I'm looking at what kind of pistons to get for it. Particularly the compression ratio. I'm looking at either 10.8:1 flat tops with 70cc heads or 9.3:1 dished pistons, btw my heads are 72 cc's. the block is decked to -5cc's I'm pretty sure. I'm ordering a stroker kit so those are some piston options, I'm thinking that the 9.3:1 pistons would be better, probably closer to 9:1 with my heads. And if I do go with the dished would I need to worry about needing valve reliefs? I've got a comp 268 cam (specs at bottom). I would like to be able to run regular fuel and run full timing, which I don't think I can do with the flat top pistons and the higher compression ratio. Thanks for the help, I'm still new to this!

Cam specs:

Duration- 268/280
Duration @.50- 224/230
Lift- .477/.480

 

[rumblefish360]

Get the 9.3-1 slugs and worry not a out the valve reliefs.

 

[FrostyMopar]

Typical valve lift for a cam like that is only about .060-.070" at TDC. Like rumble said I don't think you will have valve to piston clearance issues, especially in a stroker. But it's always wise to check with plasticine before final fitment.

Clearances should be minimum .120 on the intake side and .140 on the exhaust side.

 

[FrostyMopar]

Also, that is a very mild cam for a stroked 360. It will probably run out of puff about 4800RPM but make fantastic torque.

 

[txstang84]

By decking -5ccs, do you mean, they took off enough to remove 5cc volume? Most of the time, when talking about decking, it's 0.005" removed, or decked to zero, i.e., 9.600" on a SB. Also, running "full timing" is a matter of more than just static compression figures...quench, cam timing, vehicle weight and gearing also will determine whether or not you can run full timing-by your definition, what is full timing anyway? 34 degrees? 36 degrees? 45 degrees? If you want to make maximum power and assure proper operation of the engine itself, the distributor will likely need to be tuned as well, to match your engine's combination...it isn't just about initial, or total, but the curve and when it's "all in" that matter too.

And while that is a pretty mild cam for a 408, I guess we should ask the OP what the end goal for this engine is.

I also agree the lower compression pistons would be more suited to a street setting, but again, what's the purpose? I'd venture a guess since he wants to use pump gas, it's predominantly a street driven vehicle, but on these forums you never know.

As far as needing valve reliefs, it's not about dished or flat top, as most aftermarket and stroker kits are cut for reliefs regardless of their style since most people sourcing a stroker kit will be running larger aftermarket cams that would definitely require it. A couple more specifics would be helpful though...like what stroker kit-surely there's more than one brand of 408 kit available.

What kind of heads are you running? All you mentioned was the combustion chamber size... Do you plan to run 1.5 or 1.6 rockers? What intake/carb? Headers? Torque converter? Gears?

It's pretty safe to assume that a 9:1 408 with a mild cam would power just about anything you want to drop it into short of a dump truck, but tell us more...

 

[moper]

The way I read the original post is a 408 that need to be run on pump gas and you want to run the XE268 cam without worrying about your timing. not sure what you mean about the "-5cc" deal but I can help with some of the stuff. Tx is right - the stroker kits come with pistons that have valve reliefs.
A 4" stroke will develop a lot more compression (and cylinder pressure) than a 360 will. So the piston, head, and cam choices have to jive. I'm going to assume you have factory heads due to the "70 cc" comment.
So - the pistons will have to be dished or the static will be too high. The cam IMO is way too small, even for a static in the mid nines. You would be best served to use a dished piston, a stock type head gasket, and keep the chambers on the 72cc side. Then get a cam that's another 5-8° @ .050. That will give you a great street engine that will be able to run pump 89, maybe 87 depending on your local pump brew.

 

[Oldmanmopar]

make it bark 13-1

 

[txstang84]

Thanks moper-I guess I should've just said there are many assumptions here.

 

[1wild&crazyguy]

That cam is small for a stroker.lol

 

[DartVadar]

I was told that they were decking the block -5cc. And about the timing thing, I just don't want to have to tune because of pinging, well radically change the timing anyways. (But I could be way out to lunch with thinking that I can do that lol) and simply because it's nice to run regular fuel. And I know that cam is pretty mild, but the reason I'm wanting to use it is because I already have it, next year I'm going to buy some aluminum heads to replace the J heads that i am using currently, and then I will buy a cam that is better suited for a 408. And that's kinda what I thought with these kits, and I'm looking at the skat kits.

The main purpose of this engine is just to be fun and easy to drive on the street, it's not going to be used for racing much, if any racing at all.

Currently I have an aluminum dual plane intake, with an 750 holly, 3.55 gears and longtube headers, converter is stock currently but I'm going to change that out soon.

 

[txstang84]

I'm going to guess they probably meant 0.005" for a clean up cut. As far as 'don't want to have to tune' you may want to rethink that. A 408 will have different demands for timing than a 360 for a few different reasons, not the least of which being the displacement and stroke difference. So, as much as you don't want to mess with it, you'll still be much better off to fiddle with your timing curve to get good performance out of it. I'm not saying it won't work with a stock dizzy, but it'll run better if you tune it correctly, and if you're going to spend the money to make it a 408, then you need to spare the change to make it all work together properly.

I do have a follow up question though-if all you're going to do with this is street driving, and you don't anticipate going to the strip, or much anyway, a 360 would still get what you want accomplished. A good set of heads on a well built 360 would still propel a Dart into the 11s with the proper setup.

...or do you just want to be able to tell people "it's a stroker"?

 

[DartVadar]

I'm going to guess they probably meant 0.005" for a clean up cut. As far as 'don't want to have to tune' you may want to rethink that. A 408 will have different demands for timing than a 360 for a few different reasons, not the least of which being the displacement and stroke difference. So, as much as you don't want to mess with it, you'll still be much better off to fiddle with your timing curve to get good performance out of it. I'm not saying it won't work with a stock dizzy, but it'll run better if you tune it correctly, and if you're going to spend the money to make it a 408, then you need to spare the change to make it all work together properly.

I do have a follow up question though-if all you're going to do with this is street driving, and you don't anticipate going to the strip, or much anyway, a 360 would still get what you want accomplished. A good set of heads on a well built 360 would still propel a Dart into the 11s with the proper setup.

...or do you just want to be able to tell people "it's a stroker"?

Yea I'm sure that's what they did, and I was anticipating some tuning, that kinda stuff isn't my strong point, but I've got many other family members that are pretty good at it.

And I originally was just going to build a 360, but I needed a crank, rods and pistons anyway, so I just figured that I would go with a stroker kit because they are only a bit more expensive for the bottom end, and I have the money to build it. I also dont know what i may do with the car in the future, i may want to make it a strip car, and if I do a 408 can make more power. I would like to go to the track sometimes, but it would be mostly an street car. Also I like the torque that a stroker engine can make, and yes it's is also pretty cool to say that I have a stroker

 

[1wild&crazyguy]

I'd put an mp .528 solid in that 408 if it were me and I were looking to basically have a big inch version of a 340 that would have a similar idle and still turn 5800 if needed.
None the less I think you will have fun learning and finding out what's what.
The heads will be the limiting factor if stock, though if they are ported...you should just break in the right cam once and not twice.

 

[EL5DEMON340]

I would go with the 10.8 cr pistons, I run 11.25cr at 205 cranking psi on 93 pump. It has a great sounding POP out of the tips, starts instantly, and idles strong! I run it at 30 total.

 

[txstang84]

Just remember, when you figure out your budget for the engine build, double that number and that's closer to how much you'll spend.

 

[cudavert]

I'm with EL5DEMON340.....I like high compression, however I can get away with it. Now with these dished pistons you mentioned, does that mean there is no quench area on them? That -.005 wouldn't mean that flat top pistons would be protruding from the block deck by that much @ TDC, would it?

 

[DartVadar]

I'll maybe just go with a bigger cam then, will probably be better in the long run. And maybe the 10.8 would work, my uncle runs 10:1 pistons in his 440 and runs regular somehow...

And about the budget for this build, I was thinking about 2500-3000 for just the rotating assembly and machine work. I'm not including anything else, does that seem reasonable? I havent bought anything yet so no money lost. And I can assemble everything with my grandpa who has done this many times, so no cost in labour to put everything together.

 

[DaveBonds]

-5cc decking is not a form of measurement that I have ever heard of. I'm with txstang84 on this.

In order for a machine shop to theoretically know what -5cc would be after milling the engine block deck surface, they would have to know what piston you were using, because each piston make has a qunique shape and compression distance (distance from center/axis of wrist pin to the top of the piston), which drastically changes volume (in this case, measured in cc)

Since you haven't chosen a piston yet, I don't think any machine shop would have given you any figure, measured in cubic centimeters. Most machine shops won't even give you any figures in cubic centimeters on an SAE engine, unless you are paying them to blueprint the engine for you.

For an example static compression ratio, take the 318 I'm building;

stock 318, overbored .030" (3.940" bore diameter)

stock stroke (3.310" TDC to BDC)

stock rod length (6.123" from the center/ axis of the crank journal to the center/ axis of wrist pin)

stock deck height (9.600")

The piston I'm using has a +5cc set of valve reliefs and a .012" compression distance from the top of the piston, to the top of the block at the deck.

I'm using a head gasket that is .028" thick with a 4.140" diameter.

The heads that I am using have exactly 61cc volume, after I checked them myself.

Without calculating minimal extra volume, like the space between the piston and cylinder wall (usually .002"-.003") and without calculating the distance from the top compression ring to the top of the piston, it leaves me with a static compression ratio of 9.8:1

Dynamic ratio is down somewhere in the 8.2:1 and my actual static at this altitude is somewhere like 8.8:1 because it's at 5460ft elevation, even with 3° advance, on a 235°-240° duration (210° @ .050" lobe lift to lobe lift).

Punch the numbers you have from your parts, into a compression calculator. -

http://www.jeepstrokers.com/calculator/

That calculator has numbers in it for a 4.0L jeep in the fields, but it's the most accurate one I've found online and it only takes a second to overwrite each field.

 

[moper]

I'll maybe just go with a bigger cam then, will probably be better in the long run. And maybe the 10.8 would work, my uncle runs 10:1 pistons in his 440 and runs regular somehow...

And about the budget for this build, I was thinking about 2500-3000 for just the rotating assembly and machine work. I'm not including anything else, does that seem reasonable? I havent bought anything yet so no money lost. And I can assemble everything with my grandpa who has done this many times, so no cost in labour to put everything together.

I really think you need to do a bit more homework. You don't have enough money in that $3K for everything.
Do not run the Eagle cast crank. Pay for a Scat or MP cast.
Do not fun flat tops if you intend to replace the heads.
Do have the block Zero decked. Not ".005", not "-5ccs". It needs to be properly measured and cut to give you a piston that's flush with the deck when it's assembled.
Do replace the camshaft. Run a cheap MP if you have to - but the XE is not enough, period.
You will have to completely re-tune for the bigger stroke, regardless of the head ans camshaft choices. Be preppared.

 

[DartVadar]

So rebuilding the 360 is probably the best thing to do? Especially for my budget, as much as I would like to build a 408 I'm beginning to see that it may be more money than I'm wanting to spend on this engine. The good thing is I haven't bought anything.

I was originally quoted about 2k for zero decking (I don't know what the -5cc thing is either) pistons, turning and balancing of crank, rings and bearings, flex plate,.30 overbore, and hot tanking, I guess I thought a 408 wouldn't be too much more expensive, not enough research on my part I guess.

 

[txstang84]

You said something about needing crank/rods anyway? Why can't you use the ones in the block? Are they trashed/beyond repair?

In any case, a crank for a 360 can be had for a little bit of nothing on here or eBay-same goes for rods. They're literally everywhere.

Is the $2K the estimate for just machine work on the shortblock?

 

[DartVadar]

Well I do have a crank and rods, but they need some work, it looks kinda like whoever had the motor running before didn't change the oil much so they fairly worn and scored, I think they will work with some machining and polishing, but if not I'm not worried about finding another one for pretty cheap.

And the 2k is what they estimated for the complete shortblock all balanced and ready to be put together. Does that seem overly expensive? i know that stuff isnt cheap. They do really good quality work, my family has been going to them for years.

 

[txstang84]

Well, I don't know what your locals charge for machine work, but the last time I had work done to a V8, it was about $1100 for:

Bore
hone w/torque plates
zero decked
cleaned
crank polished (not ground)
assembly balanced
rods checked

But, I didn't need to install cam bearings in it since it's an OHC engine-installing those usually runs about $50-70 for the install/fitting, plus buying the bearings themselves, another $60. I also didn't need the crank ground, and that usually adds at least $125 to the process. So, there's a few things that I didn't have to pay for due to engine config.

That being said, if $2K is the going rate where you are for good machine work, I guess that's what you'll pay. Just recall, you'll have to buy bearings, rings, gaskets, oil pump, possibly water pump, fuel pump (if upgrading).

Do you plan on freshening the heads? That usually adds a couple hundred or so...just think, if you'd wanted a stroker, add the cost of the kit itself to all the above...

 

[DartVadar]

The 2k seems fairly reasonable then, I'm getting all the work you got done plus adding cam bearings and frost plugs. That price also included parts, pistons, rings, and all the bearings, after that I don't have to buy anything else, well actually I just need a car oil pan, but that's it. I have an new oil pump, water pump, fuel pump and a full gasket kit

My heads are rebuilt, so they are good to go, they are just the big valve iron j heads, came off an 70 something 360.

The reason why I was thinking a stroker kit was a good idea is because I already have alot of the parts, and because I wouldn't need to turn and polish the crank, buy a flex plate because if get an internally balanced kit. The machine work would probably be around a 1000 because the kit comes with everything. But I didn't take into account that better flowing heads and a bigger cam would be an immediate expense.

 

[moper]

Not so much the heads - but definately replacing the cam which may sprial into valve springs too. You'll "exchange" some parts for the kit stuff- but the kit stuff will be more expensive. and you still need all the other parts that make an engine. That's why I say I'm not sure there's enough in the kitty to get it all done. 360s are no slough anyway - I wouldn;t be too concerned - and they look identical so stick a "408" pie tin on it...lol.